Hitherto, there is known a frame of a vehicle such as a truck including a pair of side rails extending in the vehicle longitudinal direction while facing each other. Then, the side rail of the related art has a substantially C-shaped cross-section (as a channel shape) as in channel steel. However, in order to dispose a cab above a front portion of a side rail and to dispose a plurality of wiring lines between the side rail and the cab, the front portion is formed so as to be lower than a rear portion.